One method of producing sand moulds for use in the casting of metal articles is to mix together sand, binder and hardener in predetermined quantities to produce a mix which spontaneously hardens after a Preselected time to thereby produce a usable mould. A problem with prior art operations has been to select the ratios and/or types of hardener and binder to produce a mix which will harden after an accurately predictable time lapse. The predictability of the time to hardening is important for the proper running and management of a casting program. Where misjudgements are made in the mix ratios wastings of mould material or castings can result which is both undesirable and decreases the efficiency of the casting operation.
A method of controlling the time to hardening has been to produce hardener in different blends, each different blend having a different hardening time. An operator then selects the blend of hardener designed to produce a time to hardening which he requires for a particular operation. The time to hardening however also varies in accordance with the temperature of the sand, the binder and the ambient air, and thus, accurate prediction of the time a particular mix will take to harden becomes difficult. This problem is exacerbated due to the fact that the sand is usually pre-treated prior to mixing by, for example, washing and drying resulting in a wide range of sand temperatures.
A skilled operator having a reasonable range of hardener blends available to him will, with some trial and error, obtain a mix which hardens after the desired time lapse. However, changes to the sand and/or air temperature can change the hardening time, and where the desired time to hardening changes, the trial and error procedure must be redone resulting once again in wastage.
A further problem with prior art systems occurs because catalysts at opposite ends of the setting time range given by a particular family, may have different optimum addition rates with respect to the amount of binder being used in the sand. In the prior art situation, a range of pre-mixed catalyst blends would be available, consisting of two catalysts mixed in various proportions, and probably also each of those catalysts alone. A foundry would stock a sufficient number of these blends to accommodate, albeit with inconvenience, the range of conditions most likely to be experienced.
The generally used pumping mechanism in the prior art cannot be adjusted simply and reproducibly to new pumping rates. Also, the use of a moderately greater-than-optimum rate of addition of catalyst does not significantly alter the hardening characteristics of the sand but it is of course a direct waste of an expensive consumable. The use of a moderately less-than-optimum rate of addition, on the other hand, results in inadequate hardening, and is a condition to be avoided. The `optimum` rate of addition is of course that rate at which the onset of inadequate hardening is imminent.
Hence prior art systems require, because of what is practicable on the shop floor, catalyst to be added at whatever is the highest rate among the various blends that might be required; this is the only safe course to follow. It wastes material under some conditions in order not to have an inadequacy under other conditions. Clearly this is undesirable.
It is an object of this invention to provide a method of and apparatus for accurately and automatically producing a mould mix which hardens after a preselected time.
A further object of the invention is to provide means for determining and supplying an optimum ratio of catalyst mix to binder, given that a range of catalyst mixes will be used with varying conditions in use.